fix(home-assistant): clear retained old style status messages

Pre 2.18.x the status messages were retained. This isn't done anymore,
but after upgrading the old status messages are still retained on the
cluster. This just clears the old msgs to prevent any weird behavior in
the future, providing a more seamless upgrade path.

src/integrations/home-assistant/home-assistant.service.spec.ts

@@ -558,6 +558,19 @@ describe('HomeAssistantService', () => {
       }
     );
 
+    it('should clear old retained status messages from pre-2.18.x', async () => {
+      await service.onModuleInit();
+      service.handleNewEntity(new Sensor('test', 'Test'));
+
+      expect(mockMqttClient.publish).toHaveBeenCalledWith(
+        'room-assistant/sensor/test-instance-test/status',
+        '',
+        {
+          retain: true,
+        }
+      );
+    });
+
     it('should send an instance status message as heartbeat', async () => {
       await service.onModuleInit();
       mockMqttClient.publish.mockClear();

src/integrations/home-assistant/home-assistant.service.ts

@@ -171,7 +171,11 @@ export class HomeAssistantService
     this.entityConfigs.set(combinedId, config);
 
     this.sendDiscoveryMessage(config);
-    this.sendEntityStatus(config);
+
+    if (config.availability.length > 0) {
+      this.clearRetained(config.availability[0].topic); // for upgrades from < 2.18.x
+      this.sendEntityStatus(config);
+    }
   }
 
   /**
@@ -434,6 +438,15 @@ export class HomeAssistantService
     });
   }
 
+  /**
+   * Clear any retained message from the given MQTT topic.
+   *
+   * @param topic - Topic to clear ("delete")
+   */
+  private async clearRetained(topic: string): Promise<void> {
+    await this.mqttClient.publish(topic, '', { retain: true });
+  }
+
   /**
    * Publish entity status to its availability topic with MQTT.
    *